1. Field Of The Invention
The present invention relates to an oxidation catalyst formed from V.sub.2 O.sub.5 (vanadic anhydride or vanadium pentoxide) and TiO.sub.2 (titanium dioxide) of rutile structure, to the process for the preparation of such a catalyst and to the use of the same catalyst in catalytic oxidation processes, especially in the process for oxidizing o-xylene to phthalic anhydride.
2. Background Art
Oxidation catalysts consisting of vanadium and titanium oxides, which may contain an oxidation promoter, especially an alkali metal halide, are known from the state of the art.
These known catalysts can be obtained by impregnating a preformed TiO.sub.2 support of rutile structure with an aqueous solution of a decomposable vanadium salt and decomposing the vanadium compound at an elevated temperature to obtain the corresponding oxide. These catalysts show in general a low activity in oxidation processes, presumably due to the low surface area of the support. In fact, the industrial preparation of rutile takes place by calcination at very high temperatures, of the order of 900.degree. to 1,000.degree. C., of anatase TiO.sub.2 obtained by the sulfate or chloride process, and such a calcination at elevated temperatures leads to a diminution of the surface area of the rutile.
In practice, the assumption that the rutile structure of the TiO.sub.2 leads to poorer characteristics for the catalyst derives essentially from the experimental evidence that the transformation of the anatase structure into the rutile structure in the catalysts formed from vanadium and titanium oxides leads to the following undesirable phenomena: a sharp decrease in the values of the surface area, destruction of the porous structure (macropores) and formation of a solid solution of V.sup.4+ in the rutile lattice, with consequent destruction of the so-called "monolayer" of vanadium on the anatase.
These phenomena contribute to the loss of activity of the catalyst in the oxidation of o-oxylene to phthalic anhydride, and it has been reported that the formation of certain percentages of rutile in the catalyst lead to a significant reduction in the useful product yield from the reaction.
It is therefore generally preferred to use catalysts formed from oxides of vanadium and titanium containing TiO.sub.2 of anatase structure, and these catalysts can be obtained either by impregnation of preformed anatase with an aqueous solution of a vanadium salt or by coprecipitation of the hydrated titanium and vanadium oxides from a solution of the corresponding soluble salts, followed by drying of the precipitate thus obtained and calcination of the dried precipitate at elevated temperature.
In the technical and patent literature, the fact has been widely stressed that the catalysts for the oxidation of o-xylene to phthalic anhydride, based on V.sub.2 O.sub.5 and TiO.sub.2, require the anatase structure for the TiO.sub.2 namely in order to obtain active and selective catalysts, and reference is made in this respect to the descriptions by: I. E. Wachs et al., Applied Catalysis, 15 (1985) 339; J. Haber, Pure and Applied Chemistry, 56 (12) (1984) 1663; M. S. Wainwright et al., Catal. Rev.-Sci. Eng., 19 (1977)(211; and also the descriptions in U.S. Pat. Nos. 4,228,038 and 4,397,768 and German Patent No. 1,553,728.
Various theories have been advanced to explain the need to have anatase TiO.sub.2 in the catalysts under discussion, such as, for example, the presence of a crystallographic similarity between the most exposed surface planes in TiO.sub.2 and the crystallographic planes of the V.sub.2 O.sub.5 which contain the active and selective sites. Reference is made in this respect to the description by A. Vejux et al., J. Solid State Chem., 23 (1978) 93.
On the other hand, it must be remembered that the presence of vanadium in the catalyst favors the transformation of anatase TiO.sub.2 to rutile and lowers the transformation temperature from 900.degree. to 1,000.degree. C. down to 500.degree.-600.degree. C.
Therefore, in spite of accurate control of the temperature profile and the hot spots in the catalyst bed, when the catalysts based on vanadium and titanium oxides are used, a partial transformation of the anatase into rutile, with a consequent reduction in the performance of the catalyst, takes place in the course of a certain period of time.